Analysis of tensioned-web-over-slot die coating.

Abstract

In tensioned-web-over-slot die (TWOSD) coating, web is sustained only by the ten- sion of the web wrapped around the coating die. The distance between the die and web is set by the interaction between the hydrodynamic force of coating liquid and the normal stress resultant from the curved tensioned web, in what is called elaso- hydrodynamic interaction. In order to analyze this particular the coating flow, several tools are developed and tested on other coating flows relatively simpler than TWOSD flow. The theoretical conditions for the onset of vortex that degrades product qualities are proposed and the critical vortex birth trajectories inside the parameter space are tracked automatically by a direct tracking method. To detect a defect-causing flow feature in multi-layer coating, mid-gap invasion, the position of an interlayer separation point was tracked by theoretical model. The results are verified by flow visualization experiment for two- layer fixed-gap slot coating. Also linear stability analysis was performed, in order to detect unstable interlayer that signals coating defects in the two-layer slot coating. The purpose of the research is to understand the complicated flow characteristic inside the coating bead by solving the two-dimensional Navier-Stokes theory using finite element method and visualizing the coating bead flow on a lab-scale TWOSD coater. Using the tools described above, the flow features that leads to coating defects, such as, bead breakup, weeping, mid-gap invasion and feed slot vortex, are identified and mathematical forms of the onset condition for the features are presented. The onset conditions are combined into the direct tracking method that was used to construct the vortex-free operating window for the given die lip configurations. Furthermore, the tracking method can be used to shows the effect of die lip design on the critical parameters for the onset of the flow features.